I guess it's not a huge problem and it seems to go away a bit after playing for awhile (at least, I think, it's kinda inconsistent.), which is that sometimes when entering "or exiting depending on how you look at it" a cell, the game goes to the usual black screen that it does when it's about to say "loading" or after the loading screen - and stays there. Sometimes, rarely, I can alt-tab, windows button, or alt-ctrl-dlt out of it but not usually. Sometimes it hangs on the loading screen as well. The little swirly loading circle usually keeps going, I think.
I haven't had this problem with Oblivion or any other games I've played, though admittedly Fo3 is so far the hardest on my system.
I've got a P4 3.0ghz with hyperthreading, a gig ram and a Radeon HD 4650 card with 512 memory.
I've done a few tweaks, usually following the tweak guide I found (I think it's tweakguides.com or something, if you google it you'll find it.)